Device for the transmission of solar energy to a liquid medium

A device for the transmission of solar energy to a liquid medium including a flat-bottomed box having an open top, a light-permeable cover closing said top and a heat exchanger positioned within said box below said cover, said heat exchanger comprising a relatively flat platelike blow molded element having a pair of generally parallely extending spaced collecting channels formed therein and a plurality of liquid flow connecting channels integrally formed and in communication with said collecting channels, said platelike element being blow molded from an extruded tubular preform made of black pigmented polypropylene and said connecting channels having a diameter of from 5 to 25 mm, a wall thickness of from 1 to 3 mm and being spaced from each other a distance of from 2 to 30 mm.